In accordance with previous approaches, membrane surface graft polymerization has been typically achieved by initiation of graft polymerization or polymer grafting using chemical initiators in solution to initiate reactive surface sites or grafting of surface initiators. For polymeric membrane surfaces, resulting surface density of polymer chains by the above approaches can be limited by the steric hindrance associated with the binding of large molecular weight polymer chains (formed in solution) to the active membrane surface sites (i.e., polymer grafting) which would prevent a dense polymer brush layer. A variety of techniques have been developed to directly activate the membrane surface to reduce polymer grafting such as UV and gamma irradiation.
UV graft polymerization can be used for membrane surface graft polymerization but it can result in monomer initiation in solution from irradiation by the UV source. Therefore, polymer grafting is also expected, and thus a reduction in the resulting surface graft density may result. UV radiation is not as versatile as plasma activation since it is less energetic and is not universal in its applicability (i.e., a limited selection of polymeric material can be typically successfully activated by UV radiation).
Gamma irradiation has also been studied. It can be difficult to control the degree of surface activation, and the high-energy gamma irradiation involved can lead to membrane surface etching and thus alteration of the membrane permeability and possibly pin-holes. Moreover, the technique typically involves the use of a radioactive source, which reduces the commercial attractiveness of the technique, especially for large scale deployment.
Accordingly, there is a need for a polymerization technique that will allow the formation of high density membrane surface initiation sites while minimizing bulk polymer growth.